The demand for iso-alkenes has recently increased. For example, relatively large amounts of isobutene are required for reaction with methanol or ethanol over an acidic catalyst to produce methyl tert-butyl ether (MTBE) or ethyl tert-butyl ether (ETBE) which is useful as an octane enhancer for unleaded gasolines. Isoamylenes are required for reaction with methanol over an acidic catalyst to produce tert-amyl methyl ether (TAME). With passage of the Clean Air Act in the United States mandating increased gasoline oxygenate content, MTBE, ETBE and TAME have taken on new value as clean-air additives, even for lower octane gasolines. Lead phasedown of gasolines in Western Europe has further increased the demand for such oxygenates.
An article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane. The blending octane values of MTBE when added to a typical unleaded gasoline base fuel are RON=118, MON=101, R+M/2=109. The blending octane values of TAME when added to a typical unleaded gasoline base fuel are RON=112, MON=99, R+M/2=106.
The addition of shape-selective zeolite additives such as ZSM-5 to cracking catalysts, e.g., those used in fluidized catalytic cracking (FCC), is beneficial in producing gasoline boiling range product of increased octane rating. However, increased amounts of olefins result, including n-pentenes, creating a need for their conversion to higher value products such as isopentene which can be used to produce TAME.
Pentene exists in six isomers, three of which are linear, namely, 1-pentene, cis-2-pentene, and its stereo-isomer trans-2-pentene. Three isopentenes exist, 2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-1-butene, the latter not being active for TAME synthesis. Conversions between the 2-pentenes is known as geometric isomerization, whereas that between 1-pentene and the 2-pentenes is known as position isomerization, double-bond migration, or hydrogen-shift isomerization. The aforementioned three linear isomers are not branched and are known collectively as normal or n-pentenes. Conversion of the n-pentenes to the methyl-branched isopentenes is widely known as skeletal isomerization.
The reaction of tertiary olefins with alkanol to produce alkyl tertiary alkyl ether is selective with respect to iso-olefins. Linear olefins are unreactive in the acid catalyzed reaction, even to the extent that it is known that the process can be utilized as a method to separate linear and iso-olefins. The typical feedstream of FCC C.sub.5 or C.sub.5 + crackate used to produce tertiary alkyl ethers in the prior art which contains normal pentene and isopentene utilizes only the branched olefin in etherification. This situation presents an exigent challenge to workers in the field to discover a technically and economically practical means to utilize linear olefins, particularly normal butene, in the manufacture of tertiary alkyl ethers.
In recent years, a major development within the petroleum industry has been the discovery of the special catalytic capabilities of a family of zeolite catalysts based upon medium pore size shape selective metallosilicates. Discoveries have been made leading to a series of analogous processes drawn from the catalytic capability of zeolites in the restructuring of olefins.
European Patent Application 0026041 to Garwood, incorporated herein by reference, discloses a process for the restructuring of olefins in contact with zeolite catalysts having a constraint index of 1 to 12, e.g., ZSM-5 or ZSM-35, to produce iso-olefins in the presence of a diluent such as hydrogen or nitrogen, followed by the conversion of iso-olefins to MTBE and TAME. The restructuring conditions comprise temperatures between 204.degree. C. and 315.degree. C. and olefin pressures below 51 kPa.
In European Patent 0247802 to Barri et al., it is taught that linear olefins can be restructured in contact with zeolite catalyst, including Theta-1 (ZSM-22) and ZSM-23, to produce branched olefins. The restructuring conditions comprise temperature between 200.degree.-550.degree. C., pressure between 100 and 5000 kPa and WHSV between 1 and 100. Selectivities to isobutene up to 91.2% are reported using a calcined Theta-1 tectometallosilicate at 400.degree. C. and 30.6% 1-butene conversion.
U.S. Pat. No. 3,992,466 to Plank et al. teaches the use of small crystal ZSM-35 as a catalyst for hydrocarbon conversion reactions, including "isomerization of aromatics, paraffins and olefins."
U.S. Pat. No. 4,324,940 to Dessau teaches the use of shape-selective zeolites having constraint index of 2 to 12, e.g., ZSM-35, for conducting selective skeletal isomerization of olefins wherein linear olefins are preferentially reacted when in mixed streams with non-linear olefins. Conversion conditions for isomerization include 450.degree. to 1000.degree. F., 0 to 500 psig, 0.1 to 200 WHSV, and hydrogen to olefin mole ratio of 0.1 to 100.
U.S. Pat. No. 4,922,048 to Harandi discloses the use of a wide variety of medium pore size zeolites, e.g., ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35 and ZSM-48, in low temperature (232.degree.-385.degree. C.) olefin interconversion of C.sub.2 -C.sub.6 olefins to products including tertiary C.sub.4 -C.sub.5 olefins and olefinic gasoline.
U.S. Pat. No. 4,886,925 to Harandi discloses low pressure high temperature conversion of light olefins to produce higher olefins rich in isoalkenes. The process converts C.sub.2+ n-alkenes to a product comprising C.sub.4 -C.sub.6 alkenes rich in iso-alkenes, C.sub.7+ olefinic gasoline boiling range hydrocarbons, and unconverted hydrocarbons over ZSM-5. The reference teaches further treatment of the alkene effluent with methanol in the presence of medium pore size zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 and ZSM-48.
U.S. Pat. No. 4,996,386 to Hamilton, Jr. discloses concurrent isomerization and disproportionation of hydrocarbon olefins using a ferrierite/Mo/W/Al.sub.2 O.sub.3 catalyst. The catalyst exemplified produces fewer branched olefins than a comparable material free of ferrierite and the reference teaches that ferrierite-containing catalysts exhibit improved selectivity to linear olefins than conventionally prepared disproportionation catalysts.
European Patent Application 0501577 to Grandvallet et al., Barri et al. teaches the conversion of a feedstock comprising linear olefins into a branched olefin rich product over ferrierite at an olefin partial pressure of more than 0.5 bar.
European Patent Application 0523838 to Powers et al., discloses a process to convert linear alkenes to methyl branched chain alkenes using one dimensional, medium pore zeolites such as H-ferrierite or ZSM-35.
Despite the efforts exemplified in the above references, the skeletal isomerization of olefins e.g., to produce isopentenes can be hampered by the presence of impurities in feeds which are used, such as C5+ FCC and linear olefin-containing recycle feeds from etherification. Such impurities can rapidly reduce the skeletal isomerization activity of the catalyst as well as its selectivity for isopentenes.
U.S. Pat. No. 4,544,792 to Smith et al. teach hydrogen co-feed inhibits coke formation on medium pore catalysts, e.g., HZSM-5, used to convert lower olefins and oxygenates to higher hydrocarbons.
U.S. Pat. No. 4,973,790 to Beech et al. teach hydrogen co-feed inhibits coke formation on medium pore catalysts, e.g., HZSM-5, used to oligomerize lower olefins to higher hydrocarbons in the presence of basic nitrogen compounds and dienes.
All of the above references are incorporated herein by reference.